KR20140045979A - Membrane/electrode assembly for an electrolysis device - Google Patents

Abstract

The present invention provides proton-exchange membranes 401 and 402; An anode 404 and a cathode 403 disposed on both sides of the film; A conductive catalyst 410 disposed inside the proton-exchange membrane; And a conductive junction 411 connecting the catalyst 410 and the cathode 403 and having an electrical resistance greater than the proton resistance of the membrane between the catalyst and the cathode. Electrode assembly (4).

Description

Membrane / electrode assembly for electrolysis device {MEMBRANE / ELECTRODE ASSEMBLY FOR AN ELECTROLYSIS DEVICE}

The present invention relates to the production of gas by electrolysis, and in particular to an apparatus for producing hydrogen using a proton-exchange membrane for carrying out electrolysis at low temperatures of water.

Fuel cells are considered as power supply systems for many high volume applications in the future, as well as for many applications. Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. Hydrogen (H ₂ ) or hydrogen molecules are used as fuel in the fuel cell. Hydrogen molecules are oxidized at the electrode of the cell, and oxygen (O ₂ ) or oxygen molecules in the air are reduced at the other electrode of the cell. Chemical reactions produce water. A big advantage of fuel cells is that they prevent the release of air pollutants in places that generate electricity.

One of the major difficulties in developing such fuel cells is the synthesis and supply of dihydrogen (or hydrogen molecules). On earth, hydrogen is not present in large quantities except in combination with oxygen (in the form of water), sulfur (in the form of hydrogen sulfide) and nitrogen (ammonia) or carbon (fossil fuels such as natural gas or petroleum). Thus, producing hydrogen molecules requires the consumption of fossil fuels or the availability of large amounts of low cost energy in order to decompose water by thermal or electrochemical means to obtain such hydrogen.

The most prevalent method of producing hydrogen from water consists of the use of the electrolysis principle. In order to carry out this method, an electrolyzer in which a proton-exchange membrane (PEM) is provided is known. In such an electrolytic cell, the positive and negative electrodes are fixed to both sides of the proton-exchange membrane and are contacted by water. A potential difference is applied between the anode and the cathode. Thus, oxygen is produced at the anode by oxidation of water. Oxidation at the anode also generates H + ions that travel through the proton-exchange membrane to the cathode, and electrons are sent back to the cathode by the electricity supply unit. At the cathode, H + ions are reduced to the cathode level and produce hydrogen molecules.

Such electrolysis devices face unwanted effects. Thus, the proton-exchange membrane is not completely impermeable to gas. Part of the gas produced at the anode and cathode is thus passed through the proton-exchange membrane by diffusion. This not only causes a problem of purity of the gas produced but also a problem of stability. The proportion of hydrogen in oxygen must in particular be kept absolutely below 4%, which is the lower explosive limit of hydrogen in oxygen.

The permeability of the membrane to gas can be reduced by increasing the thickness of the proton-exchange membrane. However, this makes H + ions more difficult to pass, thereby increasing the electrical resistance and lowering the performance of the system.

In order to limit the permeability of the proton-exchange membrane to gas, some development methods suggest depositing catalyst particles inside the proton-exchange membrane. The catalyst particles attempt to recombine hydrogen molecules passing through the membrane with oxygen molecules passing through the membrane. The amount of oxygen molecules reaching the cathode and hydrogen molecules reaching the anode are thus reduced.

However, the recombination reaction of the catalyst particles is exothermic and causes a loss of energy. In addition, this solution is not optimized for industrial-scale applications because some of the hydrogen molecules produced at the cathode are nevertheless lost in the proton-exchange membrane. In addition, the permeability of proton-exchange membranes to hydrogen molecules is greater than its permeability to oxygen molecules. As a result, because the amount of oxygen molecules in the catalyst particles disposed in the membrane is insufficient, some of the hydrogen molecules nevertheless reach the anode.

The present invention aims to solve one or more of these disadvantages. Accordingly, the present invention provides a proton-exchange membrane; An anode and a cathode disposed on both sides of the film; A conductive catalyst 410 disposed inside the proton-exchange membrane; And a conductive junction 411 connecting the catalyst and the cathode and having an electrical resistance greater than the proton resistance of the membrane between the catalyst and the cathode. It is about.

According to one variant, the electrical resistance of the junction is at least 20 times greater than the proton resistance between the catalyst and the cathode.

According to another variant, the junction forms a perimeter that holds the proton-exchange membrane in place.

According to another variant, the junction comprises a structural part having an electrical resistivity of greater than 20 μΩcm at 293.15K.

According to another variant, the catalyst can oxidize hydrogen molecules.

According to another variant, the catalyst comprises titanium fixed to the conductive graphite support, the conductive graphite support being the first layer of the proton-exchange membrane fixedly attached to the cathode and the second layer of the proton-exchange membrane fixedly attached to the anode Is fixed to.

According to another variant, the proton resistance of the first proton-exchange layer is lower than the proton resistance of the second proton-exchange layer.

According to another variant, the proton-exchange membrane comprises a first proton-exchange layer, a second proton-exchange layer and a third proton-exchange layer, the cathode is fixed to the first proton-exchange layer, and the anode is third Fixed to the proton-exchange layer, the catalyst is a first catalyst disposed between the first proton-exchange layer and the second proton-exchange layer, and the assembly is between the second proton-exchange layer and the third proton-exchange layer And a second catalyst disposed and another conductive junction connecting the second catalyst and the anode.

The present invention also relates to an electrolysis device of water comprising the membrane-electrode assembly described above and a power supply for applying a potential difference between the anode and the cathode of the membrane-electrode assembly, the potential difference being a hydrolysis of water in contact with the anode. Suitable for

According to one variant, the resistance value of the junction between the catalyst and the cathode is set such that the voltage of the catalyst is less than 0.8V.

1 shows a cross-section of an electrolysis device incorporating a membrane-electrode assembly according to a first embodiment of the invention.
Figure 2 shows a cross section of an electrolysis device incorporating a membrane-electrode assembly according to a first embodiment of the invention.

Other features and advantages of the invention will be apparent from, but are not limited to, the following description with reference to the accompanying drawings.

The present invention proposes to place the catalyst inside the proton-exchange membrane of the membrane-electrode assembly. The electrically conductive junction connects the catalyst to the cathode by an electrical resistance of 2 to 500 times greater than the proton resistance of the membrane between the catalyst and the cathode.

The present invention makes it possible to oxidize hydrogen molecules that diffuse through the membrane from the cathode to limit the amount of hydrogen molecules reaching the anode. The present invention also results from the oxidation of hydrogen and allows hydrogen molecules to be reformed at the cathode by reducing protons with electrons collected by the catalyst. The energy efficiency of the catalyst is thus improved.

1 shows a cross section of an example of an electrolysis device 1 according to one embodiment of the invention. The electrolysis device 1 includes an electrochemical cell 2 and a power supply device 3.

The electrochemical cell 2 includes a membrane-electrode assembly 4, power supply plates 203 and 204, porous current collectors 205 and 206, and seals 201 and 202.

The membrane-electrode assembly 4 includes a proton-exchange membrane and a cathode and an anode fixed to both sides of the proton-exchange membrane. The proton-exchange membrane includes a first layer 401 to which the cathode 403 is fixed. The proton-exchange membrane includes a second layer 402 to which the anode 404 is fixed. The catalyst or catalyst layer 410 in the form of a catalyst layer is disposed inside the proton-exchange membrane between the first layer 401 and the second layer 402. The membrane-electrode assembly 4 thus comprises a stack formed by the cathode 403, the first layer 401, the catalyst layer 410, the second layer 402 and the anode 404. The membrane-electrode assembly 4 also includes an electrically conductive junction 411 connecting the cathode 403 to the catalyst layer.

The porous current collector 205 is disposed between the cathode 403 and the power supply plate 203. The porous current collector 206 is disposed between the anode 404 and the power supply plate 204.

Although not shown, power supply plate 203 has a water supply conduit connected to cathode 403 by porous current collector 205. The power supply plate 203 also has a conduit for removing hydrogen molecules, which is not shown but connected to the cathode 403 by the porous current collector 205.

Power supply plate 204 has a water supply conduit, although not shown, connected to anode 404 by porous current collector 206. The power supply plate 204 also has a conduit for hydrogen molecule removal connected to the anode 404 by a porous current collector 206 although not shown.

The power supply 3 is set to apply a DC voltage of generally 1.3 V to 3.0 V by a current density of 10 to 40000 A / m ² , advantageously 500 to 40000 A / m ² , on the power supply plate. By applying such a voltage, the oxidation of water at the anode produces oxygen molecules, and at the same time the proton reduction reaction at the cathode produces hydrogen molecules.

The reaction at anode 404 is as follows:

^{_{2H 2 O → 4H + + 4e}} - + O 2

Protons generated by the anode pass through the proton-exchange membrane to the cathode 403. The power supply 3 conducts electrons generated by the anode reaction to the cathode.

The reaction at cathode 403 is thus as follows:

2H ⁺ + 2e ^- > H ₂

The proton-exchange membrane allows a proton to move from the anode 404 to the cathode 403 and at the same time has a function of blocking not only electrons but also generated oxygen and hydrogen molecules. However, the proton-exchange membrane structure of the prior art suffers from the diffusion phenomenon caused by some of the gases produced at the cathode and anode.

The first function of the catalyst layer 410 is to oxidize the hydrogen molecules passing through the membrane to form protons. The protons thus formed return to the cathode 403 by the effect of the electric field. The amount of hydrogen molecules reaching the anode 404 is thus reduced. The second function of the catalyst layer 410 is to reduce the oxygen molecules passing through the membrane to form water. This reduction reaction in particular activates the protons present in the proton-exchange membrane.

The third function of the catalyst layer 410 is to collect electrons generated by the oxidation of hydrogen molecules and not compensated by the reduction of oxygen molecules. For this purpose, the catalyst layer 410 is conductive.

Electrons collected by the catalyst layer 410 are conducted to the cathode 403 by the conductive junction 411. Such electrons allow for further reduction of protons at the cathode 403. Thus, the production efficiency of hydrogen molecules by electrolysis is increased while at the same time the diffusion of hydrogen molecules to the anode 404 is markedly reduced.

Advantageously, the electrical resistance of the junction 411 is at least two times, advantageously at least 20 times, preferably at least 50 times, more preferably 100, than the proton resistance of the film between the layer 410 and the cathode 403. Bigger than twice By this value, generation of an excessively large leakage current is prevented.

The SHE standard potential (at 100 kPa and 298.15 K) of H ⁺ / H ₂ pairs is 0V. The SHE standard potential of the O ₂ / H ₂ O pair is 1.23 V.

Thus, the potential of layer 410 must be greater than zero to enable oxidation of hydrogen molecules and advantageously lower than 0.8 V (RHE) to ensure optimal reduction of oxygen molecules.

The permeation of hydrogen measured in a material conventionally used as a membrane corresponds to a maximum current density of 10 mA cm ⁻² (as a function of thickness and conditions such as temperature, pressure).

The current density value is the maximum value that can pass through the junction portion 411. Indeed, some of the hydrogen passing through the membrane directly recombines (reduces) oxygen with the layer 410 to form water.

The following symbols will be used:

Ucat is the cathode potential, Ra is the proton resistance between the layer 410 and the cathode 403, Rsa is the resistance of the junction 411, Sa is the cross section of the junction 411, j _jonc is the current density through the junction, Ucou Is the potential of layer 410.

Ucou-Ucat = Sa x Rsa xj _jonc , thus Ucou = Sa x Rsa xj _jonc + Ucat.

Ucou  > About 0

Ucou must be greater than -Ucat (Ucat is zero or negative). This is demonstrated if Rsa> Ra.

Ucou  <0.8 V RHE )About

Ucat is zero or negative (proton reduction potential). Therefore, for the maximum value of Ucou, that is, when Ucat = 0, Rsa must be calculated.

Thus, Ucou = Sa x Rsa xj _jonc , where Rsa = Ucou / j _jonc / Sa.

Ucou = 0.8 V (ERH), Sa = 10 cm ² And when j _jonc = 10 mA cm ^-2 , Rsa = 8 Ω is obtained.

The maximum value of the junction 411 resistance is therefore 8 Ω.

In this case, the proton resistance of the film between the layer 410 and the cathode 403 depends on its nature, its thickness and the measurement conditions (temperature, pressure), for example when the cross section of the anode 404 is 25 cm ^2. Advantageously 6 to 32 mΩ.

Finally, the electrical resistance of junction 411 is at least two times greater than the proton resistance of the film between layer 410 and cathode 403 and at most 1400 times greater than this resistance (when Ra = 6 mΩ).

The junction 411 may be obtained by a material having a high resistivity, such as a semiconducting metal oxide (eg, an oxide combined with SnO ₂ antimony or indium) or a conductive polymer. Junction 411 may be obtained by a structural element having an electrical resistivity of greater than 20 μΩcm, for example, at 293.15K. Junction 411 may also be obtained by resistive electrical construction coupled to layer 410 and cathode 403 by electrical cables. Advantageously, as shown in FIG. 1, the junction 411 forms a perimeter that holds the cathode 403 or first layer 401 in place.

Cathode 403 may advantageously be formed by using a conductive material formed by platinum particles supported by carbon.

The anode 404 can be formed by using noble metal oxides such as iridium oxide or ruthenium oxide to withstand high potentials.

Layer 410 is advantageously formed by a porous conductive support on which a catalytic material such as platinum is fixed. This layer 410 is set up in a manner known per se so that protons can pass through. Layer 410 may be obtained in the form of a conductive carbon screen to which platinum particles are fixed. Layer 410 may also be made in the form of a carbon layer coated by a platinum particle layer.

Layer 410 may be formed by using an ink comprising a catalytic material in a conductive support. The formed layer 410 may be assembled with the layers 401, 402 by any suitable method, such as high temperature compression.

Layer 410 may also be formed in the first layer 401 or the second layer 402 of the proton-exchange membrane by using such an ink. The use of the ink can be carried out by any suitable method, for example spraying, coating, silkscreen printing. The deposit of layer 410 may be obtained by other techniques such as physical vapor deposition (PVD) or metal-oxide chemical vapor deposition (MOCVD).

The thickness of layer 410 is limited, for example, so as not to cause excessive resistance to the proton diffusion through the membrane-electrode assembly 4.

Layers 401 and 402 may be formed of a material that is usually selected by one of ordinary skill in the art for a proton-exchange membrane. For example, a material such as commercially available under the trade names Nafion 211 or Nafion 212 may be used.

The permeability of proton-exchange membranes to hydrogen molecules is greater than that of oxygen molecules. The purpose is to limit hydrogen directly recombining with oxygen in layer 410. It may be desirable to use a junction 411 that can improve hydrogen permeation at the cathode 403.

The amount of oxygen present in layer 410 should be limited by the scaling of layers 401 and 402. Advantageously, the thickness of layer 402 is greater than the thickness of layer 401.

In using layers 401 and 402 made of a material commercially available under the name Nafion 211, it may be appropriate that such layers 401 and 402 have thicknesses of 25 μm and 75 μm, respectively.

In most of these cases, one would use layer 401 having a proton resistance less than the proton resistance of layer 402.

2 shows a cross section of an example of an electrolysis device 1 according to another embodiment of the invention. As in the example of FIG. 1, the electrolysis device 1 includes an electrochemical cell 2 and a power supply device 3. The power supply 3 is identical to that of the embodiment described above and will not be described in more detail.

The electrochemical cell 2 includes power supply plates 203 and 204, porous current collectors 205 and 206, and seals 201 and 202. Their structure and arrangement are the same as described with reference to FIG. 1. The electrochemical cell 2 also comprises a membrane-electrode assembly 4.

The membrane-electrode assembly 4 includes a cathode and an anode fixed to both sides of the proton-exchange membrane as well as the proton-exchange membrane. Cathode 403 and anode 404 are the same as in the embodiments described above.

The proton-exchange membrane includes a first layer 421 to which the cathode 403 is fixed. The proton-exchange membrane includes a second layer 422. The first catalyst in the form of the catalyst layer 431 is disposed inside the proton-exchange membrane between the first layer 421 and the second layer 422. The membrane-electrode assembly 4 further includes a conductive junction 441 connecting the cathode 403 to the catalyst layer 431.

The proton-exchange membrane includes a third layer 423 to which the anode 404 is fixed. A second catalyst in the form of a catalyst layer 432 is disposed inside the proton-exchange membrane between the second layer 422 and the third layer 423. The first catalyst layer 431 and the second catalyst layer 432 are thus separated from the third layer 423. The membrane-electrode assembly 4 further includes a conductive junction 442 connecting the anode 404 to the catalyst layer 432.

In this embodiment, the proton-exchange membrane has the function of passing protons of the positive electrode 404 traveling to the negative electrode 403 and at the same time blocking the generated oxygen and hydrogen molecules as well as electrons.

The catalyst layer 431 has a function of oxidizing hydrogen molecules passing through the membrane to form protons. The protons thus formed return to the cathode 403. The amount of hydrogen molecules reaching the anode 404 is thus reduced.

The catalyst layer 431 has a function of collecting electrons formed by oxidation of hydrogen molecules passing through the proton-exchange membrane. For this purpose, the catalyst layer 431 is conductive.

Electrons collected by the catalyst layer 431 are conducted to the cathode 403 by the conductive junction 441. Such electrons make it possible to further reduce protons at the cathode 403. Thus, the production efficiency of hydrogen molecules by electrolysis is increased while at the same time the diffusion of hydrogen molecules to the anode 404 is markedly reduced.

The catalyst layer 432 has a function of conducting electrons coming from the anode 404. Accordingly, the catalyst layer 432 is conductive.

Catalyst layer 432 also has the function of reducing water to pass oxygen molecules through the membrane to form water. Such a reduction reaction causes in particular the protons present in the proton-exchange membrane and the electrons produced by the oxidation of oxygen molecules at the anode 404 and conducted to the catalyst layer 432 by the conductive junction 442.

In such an embodiment, a direct reaction between the hydrogen molecules and the oxygen molecules is hardly present in the catalyst layer 431 or the catalyst layer 432 because they are separated by the second layer 422. Therefore, most of the hydrogen molecules selected through the proton-exchange membrane are oxidized before reaching the catalyst layer 432, and conversely, most of the oxygen molecules diffused through the proton-exchange membrane are reduced before reaching the catalyst layer 431. Gases that diffuse through the proton-exchange membrane are thus oxidized or reduced at the beginning of their diffusion.

The catalyst layers 431 and 432 have the same structure as the catalyst layer 410 of the above-described embodiment. Manufacturing methods equivalent to those described for catalyst layer 410 may also be used for catalyst layers 431 and 432.

Junctions 441 and 442 may have substantially the same structure as junction 411 in the embodiments described above.

The SHE standard potential (at 100 kPa and 298.15 K) of H ⁺ / H ₂ pairs is 0V. The SHE standard potential of the O ₂ / H ₂ O pair is 1.23 V.

Thus, the potential U1 of layer 431 must be greater than zero to allow hydrogen molecules to be oxidized.

The potential U2 of layer 432 should be advantageously lower than 0.8 V (RHE) to ensure optimal reduction of oxygen molecules.

The permeation of hydrogen, measured in materials conventionally used as membranes, has a maximum current density j _jonc of 10 mA cm ^-2 . Corresponds to _H2 (depending on conditions such as thickness, temperature and pressure). Mountain penetration is half of this, j _jonc Corresponds to _O2 .

The same kind of evaluation as the evaluation of the resistance value of the junction for the above-described embodiment can be performed.

Rsa is the resistance of the junction 441, Ra is the proton resistance between the layer 410 and the cathode, Rb is the proton resistance between the layer 432 and the anode, Uan is the anode potential, Ucat is the cathode potential, and Sa is the junction 441. ), Sb is defined as the cross section of the junction 442.

U1-Ucat = Sa x Rsa xj _{jonc H2}

Uan-U2 = Sb x Rsb xj _{jonc O2}

U1  > About 0

U1 must be greater than -Ucat (Ucat is zero or negative). This is demonstrated if Rsa> Ra.

Advantageously, the electrical resistance of junction 441 is greater than the proton resistance of the film between layer 421 and cathode 403. This value prevents short circuits from occurring and limits degradation of dislocations in the proton-exchange membrane.

U2  <0.8 V RHE )About

For polarization curves commonly encountered in PEM electrolysis, Uan is about 1.8 V (RHE).

Thus, for an anode voltage of 1.8 V and an Sb value of 10 cm ² , Rsb = (Uan-U2) / j _{jonc O2} / Sb, and therefore Rsb = 24Ω.

The proton resistance of the film between layer 432 and anode 404 is advantageously 6 depending on its nature, its thickness and measurement conditions (temperature, pressure), for example when the cross section of cathode 403 is 25 cm ^2. To 32 mΩ.

Finally, in this example, the electrical resistance of junction 442 is at least 750 times greater than the proton resistance of the film between layer 432 and anode 404 and at most 4000 times greater than this resistance (Rb = 32 mΩ). time).

The layers 421, 422, 423 may be made of the same material as is commonly available under the trade name Nafion 211. Here, unlike the embodiment described above, since there is no longer a direct recombination between hydrogen and oxygen in the central layer, the presence of the two junctions makes both sides independent.

As mentioned above, there is no longer a diffusion flow ratio (relative to the thickness of the layer) to be adapted between the two gases. Thicknesses of 25 μm, 25 μm and 75 μm may be proposed for layers 421, 422 and 423, respectively.

The present invention has been described with reference to an apparatus for the electrolysis of water. However, it can also be taken into account when the apparatus is set up to perform other kinds of electrolysis resulting in the production of gases, which need to prevent the diffusion of gases through the proton-exchange membrane.

Claims (10)

As a membrane-electrode assembly 4 for an electrolysis device 1,
Proton-exchange membranes 401, 402;
An anode 404 and a cathode 403 disposed on both sides of the film;
A conductive catalyst 410 disposed inside the proton-exchange membrane; And
A conductive junction (411) connecting the catalyst (410) and the cathode (403) and having an electrical resistance greater than the proton resistance of the membrane between the catalyst and the cathode. The method of claim 1,
The electrical resistance of the junction is a membrane-electrode assembly, characterized in that more than 20 times greater than the proton resistance between the catalyst and the cathode. 3. The method according to claim 1 or 2,
The junction portion 411 forms an edge frame which holds the proton-exchange membrane in place. 4. The method according to any one of claims 1 to 3,
The junction portion 411 includes a structure having a structure having an electrical resistivity of more than 20 μΩ.cm at 293.15K. 5. The method according to any one of claims 1 to 4,
The catalyst (410) is a membrane-electrode assembly, characterized in that capable of oxidizing hydrogen molecules. 6. The method of claim 5,
The catalyst 410 comprises titanium fixed to a conductive graphite support, the conductive graphite support being the first layer of the proton-exchange membrane fixedly attached to the cathode and the second layer of the proton-exchange membrane fixedly attached to the anode Membrane-electrode assembly, characterized in that fixed. The method according to claim 6,
The proton resistance of the first proton-exchange layer is lower than the proton resistance of the second proton-exchange layer. The method according to any one of claims 1 to 5,
The proton-exchange membrane includes a first proton-exchange layer, a second proton-exchange layer, and a third proton-exchange layer, wherein the cathode 403 is fixed to the first proton-exchange layer 421, and the anode 404 is fixed to a third proton-exchange layer 423, the catalyst being a first catalyst 431 disposed between the first proton-exchange layer and the second proton-exchange layer,
A second catalyst 432 disposed between the second proton-exchange layer and the third proton-exchange layer;
And another conductive junction (442) connecting said second catalyst (432) and said anode (404). A membrane-electrode assembly according to any one of claims 1 to 8, and a power supply for applying a potential difference between an anode and a cathode of the membrane-electrode assembly, wherein the potential difference is used to draw water in contact with the anode. Apparatus for electrolysis of water, characterized in that suitable for decomposition. 10. The method of claim 9,
The resistance value of the junction between the catalyst and the cathode is set so that the voltage of the catalyst is less than 0.8V (RHE).

Images